Systems and methods for projected grid-based location tracking

ABSTRACT

Systems and methods that provide a framework for location tracking of a movable target component or device (e.g., an automated device or a hand-operated device) to accurately cover an area of interest along a specified path or in a specified region. Grid patterns are projected onto a surface of a workpiece or a part. The projected grid lines may be straight or curved. Straight grid lines can be parallel or intersecting. The grid pattern may include a path to be followed. The lines of the projected grid pattern are detected by a grid detection sensor which is mounted onboard the movable target component or device. Information from the grid detection sensor is fed to a location mapping program. The systems and methods also enable navigation for use in automated and autonomous manufacturing and maintenance operations, as well as other tracking-based applications.

BACKGROUND

This disclosure generally relates to systems and methods for trackingthe locations of a movable target object (e.g., a component of anautomated apparatus or a hand-operated device that requires accuratelocation tracking) as it moves relative to a workpiece or part.

An aircraft is subjected to a variety of manufacturing and maintenanceoperations throughout its long lifetime. These operations includenon-destructive inspection, surface preparation, painting, coating,repair, cleaning, paint removal, re-painting, re-coating, among others.All of these operations would benefit from the cost, time, quality, andpersonnel safety improvements that crawling, swarming, or flyingautonomous or semi-autonomous robots can provide. While autonomoustechnology is moving in this direction, a limitation is the cost andcomplexity associated with tracking and guidance of platforms across thesurface of the structure.

Applications involving manufacturing and maintenance processes that usecrawler vehicles or other computer-controlled electro-mechanicalmachines often employ location tracking in a reference coordinatesystem. Existing location tracking solutions fit into two categories:absolute motion tracking and incremental motion tracking. Absolutemotion tracking refers to tracking of position and/or orientationdefined in a reference coordinate system, such as an airplane coordinatesystem. Known techniques for absolute motion tracking include opticalmotion capture, laser tracking, depth cameras, magnetic tracking,sonic-based tracking, and tracking using the Global Positioning System(GPS),

Incremental motion tracking measures displacements relative to a priorcoordinate measurement. One known technique for incremental motiontracking employs encoders which output pulses in response to incrementalmovements. In applications that use incremental motion measurement,errors can build up over time. Such error build-up is undesirable in usecases requiring finite error with respect to an absolute coordinatesystem. The magnitude of the error can be reduced by implementing aprocess that provides accurate, absolute measurement at lower updaterates which can be integrated into an incremental motion measurementsystem running at higher update rates.

It would be advantageous to provide a system and a method for locationtracking of a movable target object during manufacturing and maintenanceprocedures that is simple and inexpensive.

SUMMARY

The subject matter disclosed in detail below is directed to systems andmethods that provide a structured approach/framework for locationtracking of a movable target component or device (e.g., a crawlervehicle, an articulated robot arm or a hand-operated device) toaccurately cover an area of interest along a specified path or in aspecified region. These systems and methods involve the projection ofgrid patterns onto a surface of a workpiece or a part. The projectedgrid lines may be straight or curved. Straight grid lines can beparallel or intersecting. The grid pattern may include a path to befollowed. The lines of the projected grid pattern are detected by a griddetection sensor which is mounted onboard the movable target componentor device being tracked. Information from the grid detection sensor isfed to a location mapping program. The systems and methods disclosedbelow also enable a relatively simple form of navigation for use inautomated and autonomous manufacturing and maintenance operations, aswell as other tracking-based applications.

The location tracking apparatus comprises a grid pattern projection unit(also referred to herein as a “projector”) and a grid-line detectionunit (also referred to herein as a “grid-line detector”). The gridpatterns can comprise closely spaced lines that provide adequate spatialresolution or lines that are separated by large gaps that provide coarseposition, with fine position being determined using differentialodometry, encoder data, inertial sensing, etc.

Projection of the appropriate pattern onto the surface can be utilizedby an automated apparatus (hereinafter “robot”) for location trackingand verification, which significantly reduces the cost and complexity oftracking for automated/autonomous manufacturing and maintenance. Anykind of robotic device can use the technology disclosed herein forlocation tracking, including a surface crawling robot, a hovering robot,a pedestal robot having an articulated robotic arm, or an X-Y-Z scanningbridge. Grid pattern projection can also be used to track the locationof devices which are movable manually.

The grid pattern projection unit may take the form of a multi-colorprojection device that generates mutually perpendicular sets of gridlines. For this application, the pattern is projected generallyperpendicular to the surface of the workpiece or part on or over whichthe tracked device or component (automated or hand-operated) moves. Thehorizontal and vertical grid lines are disambiguated by using differentcolors. In accordance with some methods of operation, the grid patternwill be aligned with some visible features on the surface of a workpieceor part, and can be adjusted with standard projection adjustments(keystone, zoom, etc.).

The grid-line detection unit may take the form of a grid-line detectionunit having sets of photo-sensors designed to detect the differentcolors of the projected grid lines. Digital output from the grid-linedetection unit can be used to determine the X-Y position and headingangle of the tracked device or component relative to the grid pattern.The X-Y position aspect works in a similar way to a standard linearencoder by counting pulses, and the angle calculation uses sensorelement separation distances, velocity, and variation in the time ofreceipt of the pulses in order to determine heading angle. The grid-linedetection unit may further comprise boundary region sensors which aresensitive to the specific color of the projected border lines.

One aspect of the subject matter disclosed in detail below is a systemcomprising: a movable target component; a projector configured toproject a grid pattern comprising intersecting grid lines and located toproject the grid pattern toward the movable target component; agrid-line detection unit mechanically coupled to the movable targetcomponent, the grid-line detection unit comprising at least twophoto-sensors, each photo-sensor being configured to output a signalwhen an impinging grid line is detected; a data acquisition device toconvert the signals from the photo-sensors into signal data in a digitalformat; and a computer system configured to calculate a location of themovable target component based on the signal data from the dataacquisition device. In some embodiments, the computer system is furtherconfigured to control movement of the movable target component as afunction of the calculated location of the movable target component.

Another aspect of the subject matter disclosed in detail below is amethod for tracking a location of a movable target component, the methodcomprising: mechanically coupling a grid-line detection unit havingmultiple photo-sensors to the movable target component; projecting lighttoward the grid-line detection unit, the projected light being in theform of a grid pattern comprising intersecting grid lines; moving thegrid-line detection unit relative to the grid pattern; outputtingelectrical signals from the photo-sensors having characteristicsindicative of projected light of the grid pattern sensed by individualphoto-sensors during movement of the grid-line detection unit; andtracking a current location of the grid-line detection unit relative tothe grid pattern using digital data derived from the electrical signalsoutput by the photo-sensors. This method may further comprise computinga current location of the movable target component relative to a frameof reference of a workpiece or part based on at least a location of thegrid pattern relative to the frame of reference of the workpiece or partand a current location of the grid-line detection unit relative to thegrid pattern.

In accordance with some embodiments, the method described in thepreceding paragraph further comprises: comparing the current location ofthe movable target component to a target location of the movable targetcomponent; causing the movable target component to move toward thetarget location; mechanically coupling a tool to the robotic apparatus;and using the tool to perform a manufacturing or maintenance operationon the workpiece or part while the component of the robotic apparatus islocated at the target location. In accordance with other embodiments,the method described in the preceding paragraph further comprises:equipping the movable target component with a non-destructive inspectionprobe; using the non-destructive inspection probe to acquirenon-destructive inspection data from the workpiece or part while themovable target component is at its current location; and mapping theacquired non-destructive inspection data to location data representingthe current location of the movable target component in a non-transitorytangible computer-readable medium.

A further aspect is an apparatus comprising a movable target component,a tool mechanically coupled to the movable target component, and agrid-line detection unit mechanically coupled to the movable targetcomponent, wherein the grid-line detection unit comprises a substrate,first through fourth photo-sensors that detect light having a firstcolor and do not detect light having a second color different than thefirst color, and fifth through eight photo-sensors that detect lighthaving the second color and do not detect light having the first color,the first and second photo-sensors being disposed adjacent to each otheron a surface of the substrate and away from the other photo-sensors, thethird and fourth photo-sensors being disposed adjacent to each other onthe surface of the substrate, away from the other photo-sensors andopposite to the first and second photo-sensors, the fifth and sixthphoto-sensors being disposed adjacent to each other on the surface ofthe substrate and away from the other photo-sensors, and the seventh andeighth photo-sensors being disposed adjacent to each other on thesurface of the substrate, away from the other photo-sensors and oppositeto the fifth and sixth photo-sensors. In some embodiments, the substratehas first through fourth corners, and the grid-line detection unitfurther comprises ninth through twelfth photo-sensors that detect lighthaving a third color different than the first and second colors and donot detect light having the first and second colors. In suchembodiments, the first through eighth photo-sensors do not detect lighthaving the third color.

Other aspects of systems and methods for location tracking of movabletarget components are disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, functions and advantages discussed in the precedingsection can be achieved independently in various embodiments or may becombined in yet other embodiments. Various embodiments will behereinafter described with reference to drawings for the purpose ofillustrating the above-described and other aspects.

FIG. 1A is a diagram representing an end view of a gridprojection/detection system for tracking a crawler robot that is movingon a surface of a workpiece or part that is relatively horizontal.

FIG. 1B is a diagram representing an end view of a gridprojection/detection system for tracking a crawler robot that is movingon a surface of a workpiece or part that is relatively vertical.

FIG. 2 is a diagram representing a side view of a gridprojection/detection system for tracking a head of a robotic arm that ismoving relative to a surface of a workpiece or part.

FIG. 3 is a diagram representing a projected grid pattern consisting ofvertical grid lines in a first color, horizontal grid lines in a secondcolor, and boundary lines in a third color.

FIG. 4A is a diagram representing a projected grid pattern with anon-rectangular boundary in which the boundaries include steps, an angleand an internal stay-out zone.

FIG. 4B is a diagram representing a projected grid pattern with anon-rectangular boundary in which the boundaries include arcs, an angleand an internal stay-out zone.

FIGS. 5A through 5D are diagrams representing a top view of asolid-state grid-line detection unit in accordance with one embodiment.These diagrams depict successive positions of the grid-line detectionunit as it moves vertically upward with a rotation angle of zerorelative to a pair of stationary mutually perpendicular grid lines whichare being projected onto a workpiece or part.

FIG. 5E is a diagram indicating the timing of pulses generated by twopairs of photo-sensors of the grid-line detection unit during movementof the grid-line detection unit relative to the grid lines as depictedin FIGS. 5A through 5D.

FIGS. 6A through 6D are diagrams representing a top view of a grid-linedetection unit in accordance with one embodiment. These diagrams depictsuccessive positions of the grid-line detection unit as it movesvertically upward with a non-zero rotation angle relative to a pair ofstationary mutually perpendicular grid lines which are being projectedonto a workpiece or part.

FIG. 6E is a diagram indicating the timing of pulses generated by twopairs of photo-sensors of the grid-line detection unit during movementof the grid-line detection unit relative to the grid lines as depictedin FIGS. 6A through 6D.

FIGS. 7A and 7B are diagrams representing a top view of a grid-linedetection unit having boundary sensors in accordance with anotherembodiment. These diagrams depict successive positions of the grid-linedetection unit as it moves rightward with a rotation angle of zerorelative to a stationary boundary line which is being projected onto aworkpiece or part.

FIG. 8 is a diagram representing a top view of a grid-line detectionunit designed for detecting large angles of rotation in accordance witha further embodiment.

FIGS. 9A through 9C are diagrams representing respective top views of acrawler robot during a motion sequence involving forward motion (FIG.9A), rotation (FIG. 9B), and boundary detection (FIG. 9C).

FIG. 10 is a block diagram showing an overall architecture of anautomated system that incorporates robotic location tracking navigationapparatus in accordance with one embodiment.

FIG. 11 is a diagram representing top view of some components of a knownencoder-equipped hand-operated device whose incremental movements can betracked using a dead-reckoning odometry-based process.

FIG. 12 is a diagram representing an isometric view of anoscillating/rotating grid-line detection unit having two photo-sensorsfor grid-line detection in accordance with one alternative embodiment.

FIG. 12A is a diagram representing a top view of the grid-line detectionunit depicted in FIG. 12.

FIG. 12B is a diagram representing a top view of the grid-line detectionunit depicted in FIG. 12 at an instant in time when two projected gridlines are respectively aligned with the linear photo-sensors.

FIG. 12C is a diagram representing a top view of the grid-line detectionunit depicted in FIG. 12B after the robot has rotated relative to theprojected grid pattern.

FIG. 12D is a diagram representing a top view of the grid-line detectionunit depicted in FIG. 12C at an instant in time duringoscillation/rotation when two projected grid lines are respectivelyaligned with the linear photo-sensors.

FIG. 13 is a diagram representing a top view of a grid pattern thatincorporates a track for guiding a crawler robot or robotic arm around astructural feature to be avoided.

FIG. 14 is a diagram representing a top view of a grid pattern thatincorporates a track for guiding a crawler robot or robotic arm around astructural feature to be avoided.

Reference will hereinafter be made to the drawings in which similarelements in different drawings bear the same reference numerals.

DETAILED DESCRIPTION

Illustrative embodiments of a location tracking system are described insome detail below. However, not all features of an actual implementationare described in this specification. A person skilled in the art willappreciate that in the development of any such actual embodiment,numerous implementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

For the purpose of illustration, location tracking for the purpose offeedback control of robotic apparatus such as crawler robots andarticulated robot arms will be described in detail below. However, theconcept of using a projected grid pattern to track the location of amovable target component can also be used for measurement (mapping)purposes. The latter mapping application encompasses location trackingof hand-operated devices as well as location tracking of components ofautomated apparatus such as robots.

FIG. 1A depicts a grid projection/detection system for tracking acrawler robot 10 that is moving on a relatively horizontal surface 22 ofa workpiece or part. FIG. 1B shows the same grid projection/detectionsystem for tracking the crawler robot 10 when it is moving on arelatively vertical surface 12 of a workpiece or part.

As seen in FIGS. 1A and 1B, the grid projection/detection systemcomprises a grid pattern projector 14 (hereinafter “projector 14”) thatprojects a grid of lines toward the surface of the workpiece or part anda grid-line detection unit 16 that detects when a projected grid lineimpinges on any photo-sensor (not shown in FIGS. 1A and 1B) of thegrid-line detection unit 16.

Patterns are projected into the space that the crawler robot 10functions in, and picked up by photo-sensors of the grid-line detectionunit 16 which is mounted on the crawler robot 10. These patterns can bemade of visible light from a laser projector or standard (non-laser)color video projector, IR or UV, or other non-visible light. The crawlerrobot 10 can have encoder wheels or an inertial sensor to assist inincremental tracking when the photo-sensors are between grid lines.Information from the grid-line detection unit 16 is fed into aposition/orientation mapping program that is hosted on a computer (notshown in FIGS. 1A and 1B), such as an expert workstation at anoperational command center. The pattern can have optional locatingfeatures that can be positioned over known features (such as feature 18in FIG. 1B) on the workpiece or part (by expanding, rotating, ortranslating the projection until there is a fit with known locations)for direct correlation of the pattern to a CAD model of the workpiece orpart.

The grid-line detection unit 16 is oriented generally toward theprojector 14. The pattern can be sensed using any one of various imagesensors (a photo-sensor or photo-detector array, a CCD image sensor, aCMOS image sensor, etc.).

Multiple grid-line detection units can work simultaneously within thegrid projection area. In some embodiments, each vehicle or device mayhave its own grid-line detection unit and work independently of theothers. In other embodiments, the vehicles or devices may be part of alarger centralized system.

FIG. 2 depicts a grid projection/detection system for tracking thetarget component 28 (e.g., an end effector) of a robot 20 as the targetcomponent 28 moves across a relatively vertical surface 12 of aworkpiece or part. The target component 28 may be mounted on a distalend of an articulated robotic arm 26, a proximal end of the robotic arm26 being attached to a pedestal 24. The grid projection/detection systemmay be the same system previously described with reference to FIGS. 1Aand 1B.

In the scenario depicted in FIG. 2, patterns are projected by theprojector 14 into the space that the target component 28 functions in,and picked up by a grid-line detection unit 16 mounted on the targetcomponent 28. Information from the grid-line detection unit 16 is fedinto a position/orientation mapping program.

As described previously, the projected grid patterns can be made ofvisible light from a laser projector, IR or UV, or other non-visuallight. In some embodiments, pulsed light can be used. For example, thesystem may comprise three lasers, with the same color, each having adifferent pulsed frequency that the grid-line detection unit 16 woulddetect. The signal-to-noise ratio would be better if looking forspecific frequencies from a single red laser that the photocell could bemore sensitive too as opposed to various color lasers. This would alsobe useful in bright sunlight when trying to detect with thephotosensitive elements.

In accordance with some embodiments of the apparatus proposed herein,the grid pattern of colored lines is generated by a multi-colorprojector. In many cases the pattern should be projected approximatelyperpendicular to the surface of the workpiece or part on or over whichthe robot will move. For calibrated tracking, the pattern may need to bealigned with some visible features on the target surface, andpotentially adjusted with standard projection adjustments based onmeasured or modeled geometry and projector position and orientation.

One concept presented here uses a regularly spaced projected pattern forreal-time tracking, as part of a navigation system forautomated/autonomous systems. One basic pattern comprises two mutuallyperpendicular sets of mutually parallel grid lines wherein thehorizontal and vertical grid lines are projected in different colors. Ina preferred embodiment, the system includes a projector capable ofprojecting in at least three colors: red, green, and blue. The grid-linedetection unit 16 has photosensitive elements that are sensitive tospecific colors. The three projected colors are associated withdifferent lines of the projected grid pattern 30 and will be used by thecontrol system to provide position and orientation data as well asboundary regions.

FIG. 3 is a diagram representing a projected grid pattern 30 consistingof straight vertical grid lines 32 in a first color (e.g., green),straight horizontal grid lines 34 in a second color (e.g., red), andstraight boundary lines 36 a-36 d that form a border 36 in a third color(e.g., blue). In this configuration, the border 36 is a rectangle, thevertical grid lines 32 are evenly spaced and extend from boundary line36 a to boundary line 36 c, and the horizontal grid lines 34 are evenlyspaced and extend from boundary line 36 b to boundary line 36 d.

The projected pattern layout is not limited to having a purelyrectangular boundary. Boundaries may include one or more of thefollowing: angles, arcs, and internal lines bounding a region where therobot should stay out. (Such a bounded region will be referred tohereinafter as a “stay-out zone”.) FIG. 4A shows a projected gridpattern 40 with a complex boundary in which the border 36A includessteps, an angle and a rectangular internal stay-out zone 44. Theprojected grid pattern 40 with a complex boundary further comprisesvertical grid lines 32 and horizontal grid lines 34 which start and endat the border 36A. FIG. 4B shows a projected grid pattern 42 with acomplex boundary in which the border 36B includes arcs, an angle and atriangular internal stay-out zone 46. The projected grid pattern 42 witha complex boundary further comprises vertical grid lines 32 andhorizontal grid lines 34 which start and end at the border 36B.

More complex boundary configurations, such as a branching passages andmazes, are also feasible. Also, although the spacing between thehorizontal grid lines 32 is depicted in FIGS. 3, 4A and 4B as beingequal to the spacing between the vertical grid lines 34, those spacingsneed not be equal. A ratio of unequal horizontal and vertical spacingscould be specified to enable different spatial resolutions along therespective axes.

The other major part of the system is a photo-sensitive device (i.e.,the grid-line detection unit 16) capable of detecting impinging gridlines and other elements of the projected pattern. The digital outputfrom the grid-line detection unit 16 can be used to determine the X-Yposition and heading angle of the robot relative to the pattern, as wellas other signals activated by the pattern (such as boundaryindications).

FIGS. 5A through 5D are diagrams representing a top view of asolid-state grid-line detection unit 16A in accordance with oneembodiment. FIGS. 5A-5D show successive positions of the grid-linedetection unit 16A as it moves vertically upward relative to a pair ofprojected mutually perpendicular grid lines 32 and 34 which arestationary. As the grid-line detection unit 16A moves upward, therelative vertical position where grid lines 32 and 34 impinge on thegrid-line detection unit 16A moves downward in the frame of reference ofthe grid-line detection unit 16A. In this example, the angle of rotationof grid-line detection unit 16A relative to the grid lines 32 and 34 iszero (i.e., the grid-line detection unit 16A is moving parallel to gridline 32).

In the embodiment shown in FIGS. 5A-5D, the projected light that formshorizontal grid line 34 is one color (e.g., red), while the projectedlight that forms vertical grid line 32 is a different color (e.g.,green). The grid-line detection unit 16A in turn comprises two pairs ofred photo-sensors (i.e., photo-sensor pairs 64 and 68) which detect onlyred light and two pairs of green photo-sensors (i.e., photo-sensor pairs62 and 66) which detect only green light. In the example depicted inFIGS. 5A-5D, the green grid line 32 does not impinge upon the greenphoto-sensors of pairs 62 and 66, but as shown in FIGS. 5B and 5C, thered grid line 34 impinges on the red photo-sensors of pairs 64 and 68 asthe grid-line detection unit 16A moves upward. The photo-sensors areattached to or formed in a substrate 96, which may be a printed circuitboard.

In cases where the substrate 96 of the grid-line detection unit 16A issquare or rectangular, the photo-sensor pairs 62, 64, 66 and 68 arepreferably located at the midpoints of the respective sides of thesquare or rectangle proximate to an edge of the substrate 96. Thephoto-sensors of each pair 62, 64, 66, 68 are arranged close together todetect the direction of motion in a way which is similar how a standardquadrature positional/rotational encoder works. Existing hardware toread quadrature encoder signals can be used with this grid-linedetection unit. As a grid line passes over a quadrature photo-sensor,either an X or a Y signal is recorded. Two photo-sensors are located inclose proximity to each other, with a slight positional offset. The pairof sensors produce signals that are slightly offset, which are processedtogether (i.e., quadrature mode); this allows the direction of motion tobe determined. Separate colors, along with corresponding sensorssensitive to the respective colors, are used for the horizontal andvertical grid lines to isolate the X and Y signals.

In the example depicted in FIGS. 5A-5D, the grid line 34 is depicted infour respective positions relative to the photo-sensor pairs 64 and 68as the grid-line detection unit 16A moves vertically upward relative tothe grid line 34. For the purpose of the following discussion, the upperand lower photo-sensors of pair 68 have been respectively designated asred photo-sensor 1A and red photo-sensor 1B in FIGS. 5A-5D. Similarly,the upper and lower photo-sensors of pair 64 have been respectivelydesignated as red photo-sensor 2A and red photo-sensor 2B.

In FIG. 5A, red grid line 34 is shown at a distance from and notimpinging upon either of red photo-sensors 1A and 2A (no pulses areoutput by the red photo-sensors). In FIG. 5B, red grid line 34 is shownconcurrently impinging upon red photo-sensors 1A and 2A. As a result ofthe projection of grid line 34 onto red photo-sensors 1A and 2A, redphoto-sensors 1A and 2A concurrently output the respective pulsesdepicted in an idealized fashion and identified by “1A” and “2A” in FIG.5E. In FIG. 5C, red grid line 34 is shown concurrently impinging uponred photo-sensors 1B and 2B. As a result of the projection of red gridline 34 onto red photo-sensors 1B and 2B, red photo-sensors 1B and 2Bconcurrently output at a subsequent time the respective pulses depictedin an idealized fashion and identified by “1B” and “2B” in FIG. 5E. InFIG. 5D, red grid line 34 is shown at a distance from and not impingingupon either of red photo-sensors 1B and 2B (no pulses are output by thered photo-sensors).

FIG. 5E indicates the timing of the pulses generated by the redphoto-sensor pairs 64 and 68 during the movement of grid-line detectionunit 16A which is depicted in FIGS. 5A-5D. The quadrature signalsdepicted in FIG. 5E associated with photo-sensor pair 1A-1B and pair2A-2B are output to a data acquisition device, which converts the pulsesinto digital data in a format acceptable to a computer that isprogrammed to process the digital pulse information. More specifically,the computer can be configured to convert the digital pulse informationinto the Y coordinate of a reference location (such as the center point)on the grid-line detection unit 16A relative to the coordinate frame ofreference of the workpiece or part at the time when the red grid line 34passed over the red photo-sensor pairs 64 and 68. In a similar manner,the quadrature signals output by the pairs green photo-sensors(photo-sensor pair 62 at the top and photo-sensor pair 66 at the bottom)when the green light of a vertical grid line impinges thereon duringhorizontal movement of the grid-line detection unit 16A can be processedto determine the X coordinate at the time when the green grid line 32passed over the green photo-sensor pairs 62 and 68. In order to makethese calculations, the computer uses a pre-stored transformation matrixthat transforms the coordinates of the grid-line detection unit 16A inthe frame of reference of the projected grid pattern into coordinates inthe frame of reference of the workpiece or part. This transformationmatrix can be calculated after the projected grid pattern has been tiedto known structural features of the workpiece or part, the structure ofwhich can be represented by a CAD model stored in a non-transitorytangible computer-readable storage medium.

The X-Y tracking part of the foregoing process is similar in concept toearly optical mice which used grid lines printed on a rigid mouse pad.One difference in this application is the use of a projected pattern andadditional sensor pairs that enable the determination of the angle ofthe grid-line detection unit 16A relative to the grid lines 32 and 34(for angles that do not approach 90 degrees).

FIGS. 6A through 6D show successive positions of the grid-line detectionunit 16A as it moves vertically upward relative to grid lines 32 and 34with a non-zero rotation angle θ. As the grid-line detection unit 16Amoves upward, the red grid line 34 impinges on the red photo-sensors ofpairs 64 and 68 in the following order: (1) red photo-sensor 1A (FIG.6A); (2) red photo-sensor 1B (FIG. 6B); (3) red photo-sensor 2A (FIG.6C); and (4) red photo-sensor 2B (FIG. 6D). FIG. 6E is a diagramindicating the timing of the quadrature signals generated by the redphoto-sensor pair 1A and 1B, and pair 2A and 2B during relative movementof the red grid line 34 as depicted in FIGS. 6A through 6D. Theresulting quadrature signals output by the red photo-sensor pair 1A and1B, and pair 2A and 2B during this movement can be used to determine therotation angle θ.

The rotation angle θ (shown in FIG. 6A) of the grid-line detection unit16A is computed by knowing its velocity V, the red photo-sensor pairseparation distance d (shown in FIG. 6A), and the time difference Δt(shown in FIG. 6E) between the pulses output by the red photo-sensors 1Aand 2A as the red grid line 34 passes over those red photo-sensors onopposite sides of the grid-line detection unit 16A. The velocity V canbe computed in a well-known manner based on time and distance. Theequation for computing the rotation angle θ is:θ=a tan(VΔt/d)The angle can be computed by using either the pairs of red photo-sensors(64 and 68) or the pairs of green photo-sensors (62 and 66).

The grid-line detection unit 16A depicted in FIGS. 5A-5D and 6A-6D canbe modified to include boundary line photo-sensors that are sensitive toa different color than the colors detected by the red and greenphoto-sensors. In the example shown in FIGS. 7A and 7B, bluephoto-sensors 72 a-72 d that are sensitive to blue light are placed atthe corners of a modified grid-line detection unit 16B, and areactivated when the blue light of projected boundary line 36 b impingeson the blue photo-sensor.

FIGS. 7A and 7B depict successive positions of the grid-line detectionunit 16B as it moves leftward with a rotation angle of zero relative toa stationary border 36 which is being projected onto a workpiece orpart. Only portions of impinging blue border 36 are shown in FIGS. 7Aand 7B, to wit, a portion of horizontal boundary line 36 a and a portionof vertical boundary line 36 b. In FIG. 7A, the grid-line detection unit16B is located such that the projected blue boundary lines 36 a and 36 bdo not overlie any of the blue photo-sensors 72 a-72 d. The grid-linedetection unit 16B is then moved leftward until the blue light ofprojected boundary line 36 b impinges on blue photo-sensors 72 a and 72d, as depicted in FIG. 7B. In response to that event, blue photo-sensors72 a and 72 d output respective pulses to a data acquisition device,which converts the pulses into digital data in a format acceptable tothe computer. In response to receiving digital pulse informationindicative of the fact that the blue boundary line 36 b—representing theleftmost limit of where the robotic apparatus should be operating—isaligned with the blue photo-sensors 72 a and 72 d, the computer cancontrol the robotic apparatus so that it does not move further to theleft.

When the rotation angle of the grid-line detection unit approaches 90degrees, the time difference Δt (shown in FIG. 6E) between the pulsesoutput by the red photo-sensors 1A and 2A as the red grid line 34 passesover approaches infinity and the angle analysis capability of the systemwould not work properly. If the application requires that the grid-linedetection unit address larger rotation angles, one solution is to haveboth types of color-sensitive photo-sensors (e.g., red and green) ateach location, as shown in FIG. 8.

The angle measurement process also becomes impractical for real-timeusage if it gets too large (like a few seconds). Also, there is arelationship between d and the grid spacing distance D that may become aproblem before the angle gets to 90 degrees. The reason for this is thatsignals from consecutive grid lines can be misinterpreted. If d isconsiderably less than D(d<<D), then the angle at which themisinterpretation problem occurs is close to 90 degrees, but if d issimilar in size to D(d≈D), then a problem may occur closer to 45degrees. In general, the constraint equation is: d<D cos θ.

FIG. 8 is a top view of a grid-line detection unit 16C having four bluephoto-sensors 72 a-72 d located in respective corners, eight greenphoto-sensors 74 a-74 h, and eight red photo-sensors 76 a-76 h. Thegreen and red photo-sensors are disposed along outer and inner circles80 and 82 indicated by dashed curved lines in FIG. 8. The greenphoto-sensors 74 a and 74 b are disposed adjacent to each other alongthe outer circle 80 and midway between the blue photo-sensors 72 a and72 b, while the red photo-sensors 76 a and 76 b are respectivelydisposed along the inner circle 82 adjacent to green photo-sensors 74 aand 74 b. Similarly, the green photo-sensors 74 e and 74 f are disposedadjacent to each other along the outer circle 80 and midway between theblue photo-sensors 72 c and 72 d, while the red photo-sensors 76 e and76 f are respectively disposed along the inner circle 82 adjacent togreen photo-sensors 74 e and 74 f. Conversely, the red photo-sensors 76c and 76 d are disposed adjacent to each other along the outer circle 80and midway between the blue photo-sensors 72 b and 72 c, while the greenphoto-sensors 74 c and 74 d are respectively disposed along the innercircle 82 adjacent to red photo-sensors 76 c and 76 d. Similarly, thered photo-sensors 76 g and 76 h are disposed adjacent to each otheralong the outer circle 80 and midway between the blue photo-sensors 72 aand 72 d, while the green photo-sensors 74 g and 74 h are respectivelydisposed along the inner circle 82 adjacent to red photo-sensors 76 gand 76 h. In accordance with some embodiments, during operation thetracking processing/control system would switch which set ofphoto-sensors are active. For example, the outer set of photo-sensors(i.e., the photo-sensors disposed along the outer circle 80) may beactive normally, and then the system switches to the inner set (i.e.,the photo-sensors disposed along the inner circle 82) when the headingangle is greater than +45 degrees (or less than −45 degrees).

For the purpose of illustrating one category of target components, FIGS.9A through 9C present respective top views of a crawler robot 10 duringa motion sequence involving forward motion in a direction parallel tothe vertical grid lines 32 (FIG. 9A), rotation (FIG. 9B), and boundarydetection (FIG. 9C). The crawler robot may be a holonomic-motioncrawler. However, the location tracking systems disclosed herein areequally applicable to operation of non-holonomic-motion crawlervehicles. Other categories of movable target components—the locations ofwhich can be tracked using the technology disclosed herein—includecomponents of other types of automated devices, such as articulatedrobots, and components of devices which are moved manually, not byautomation.

FIG. 10 is a block diagram showing an overall architecture of anautomated system that incorporates robotic location tracking navigationapparatus in accordance with one embodiment. The system depicted in FIG.10 comprises a robot component 50 (e.g., a tool-holding frame of acrawler robot or a tool-holding end effector frame of a robotic arm)that supports a manufacturing or maintenance tool 60. Movement of therobot component 50 and operation of the tool 60 can be controlled by acomputer 48. The computer 48 may comprise a general-purpose computerprogrammed with motion control application software comprisingrespective software modules for controlling movement of robot component50 and operation of tool 60.

In accordance with the embodiment depicted in FIG. 10, a grid-linedetection unit 16 is attached to the robot component 50 forintermittently providing information regarding the current location ofthe robot component 50. For example, the robot component 50 may comprisea head or an end effector attached to a distal end of a robotic arm or atool-holding frame of a crawler robot, to which the tool 60 is attached.During a manufacturing or maintenance operation, the robot component 50will be placed in contact with or adjacent to a surface of a workpieceor a part (e.g., an airplane fuselage section). The grid projector 14projects a grid pattern onto the surface of the workpiece or part in thearea where the manufacturing or maintenance operation is to be performed(i.e., by the tool 60). Several types of commercial off-the-shelf (COTS)projectors are available that would be compatible, including a standardvideo projector, a multi-color laser pattern projector, and amulti-color scanning laser video projector. The grid-line detection unit16 comprises multiple pairs of photo-sensors which output pulses inresponse to detection of impinging light of grid lines projected by theprojector 14 during movement of the robot component 50. Those pulses areoutput to a data acquisition device 56, which converts the pulses intodigital data in a format acceptable to a computer 48. The dataacquisition device 56 may comprise an existing COTS device for readingencoders and digital inputs which has encoder inputs, digital input andoutputs, and a computer interface, such as USB, serial, Ethernet, etc.

To enable the grid-line detection unit 16 to track the location of therobot component 50 in the coordinate frame of reference of the workpieceor part, the grid pattern must be initially aligned with the workpieceor part. In the simplest cases involving a flat surface and rectangularprojection area, the grid pattern can be tuned to have a specific numberof lines in the X and Y directions. This is accomplished using aprojector with an adjustable field-of-view (zoom), keystone adjustment,as well as the ability to adjust the grid pattern. By knowing the gridline spacing and the absolute location of the starting X and Y edgelocations of the grid pattern, defined in terms of the coordinate frameof reference of the workpiece or part, the location of the grid-linedetection unit 16 can be determined at run-time in the coordinate frameof reference of the workpiece or part. In other cases where sufficientrectangular (orthogonal) reference edges do not exist on the workpieceor part, the initial projection calibration process can be a separatestep.

The pattern used for calibration does not need to be the same as thegrid pattern. If the relative position and scale of the calibration andgrid patterns is known, the calibration pattern can be used for setupand then the pattern can be switched to the grid pattern for run-timeoperation. With this type of calibration, a calibration pattern may beprepared specifically for the target area on which the pattern will beprojected. For example, a calibration pattern can be created thatincludes three or more non-collinear points that represent known visiblefeatures on the workpiece or part. The user then adjusts the aimdirection, field-of-view, and keystone of the projector 14 to align theprojected points with the known features on the workpiece or part. Whenthe calibration is complete, the user switches over to the grid pattern(the calibration points are no longer shown).

In a variation of this concept, an outline shape of the workpiece orpart or some other visible portion of the workpiece or part can be usedalong with the pattern boundary lines for the alignment. In othervariations of the concept, multiple calibration patterns can be preparedand one selected during the calibration setup to allow more flexibilityduring calibration. For example, a 3-D model of the workpiece or partrendered in an interactive 3-D visualization environment can be used asthe image source for alignment calibration.

In another embodiment, a separate calibration system (not shown in FIG.10) can be used, such as a local positioning system (or other type oflaser tracker). In this process the user would use the local positioningsystem (LPS) to target and measure three feature points, with knownX,Y,Z positions, that are visible on the workpiece or part, and thencalibrate the LPS instrument to those points using a three-point methodcalibration process. In this embodiment, the projector would be attachedto the LPS pan-tilt unit with a known offset (e.g., a 4×4 transformationmatrix). After the LPS is calibrated, the offset is applied to the LPScalibration, resulting in calibration of the projector 14 to theworkpiece or part, and by knowing the field-of-view of the projector 14,the scale of the projected grid pattern on the target can be adjusted.

Projectors exist that can produce grid lines based on programmablepatterns, like the kind of controllable laser units used for laser lightshows, but a more general solution is to use a laser video projectorthat can receive video data from a standard personal computer. Thisallows applications that can create standard image data to be used asthe source, and makes it easier to generate the custom grid patternsneeded for use with non-flat surfaces.

After the projector 14 has been calibrated to the workpiece or part, therobot component 50 is placed at a starting position such that thegrid-line detection unit 16 has a known location within the projectedgrid pattern. This can be accomplished by placing the grid-linedetection unit 16 so that a selected intersection of a vertical gridline and a horizontal grid line of the projected grid pattern issuperimposed on a selected point (e.g., a reference point or mark on theunit) on the grid-line detection unit 16 and then rotating the grid-linedetection unit 16 so that a selected grid line is superimposed on twoselected points (e.g., two reference points) on the grid-line detectionunit 16.

Once the initial location of the grid-line detection unit 16 relative tothe grid pattern has been established, since the location of the gridpattern in the coordinate frame of reference of the workpiece or part isknown, the initial location of the grid-line detection unit 16 in thecoordinate frame of reference of the workpiece or part can becalculated. Thereafter, any movement of the robot component can betracked based on the change in location of the grid-line detection unitrelative to its initial location. This provides accurate, absolutemeasurement at discrete intervals.

The grid-based tracking can produce accurate location data at the gridlines, but does not produce data between the grid lines. In the case ofa crawler robot, this can be addressed by using a combination of thegrid-based tracking, differential odometry, and a filtering process(such as a Kalman filter) to fill in the gaps. This would enableabsolute measurement at lower update rates to be integrated with anincremental motion measurement system running at higher update rates.

One example of an incremental motion measurement system is adead-reckoning odometry-based system. In another embodiment, an inertialmeasurement unit can be used to provide the incremental motionmeasurement for the dead-reckoning process. Any dead-reckoning solutionwill have measurement inaccuracies due to small errors that build upover time. These can be caused by systematic errors in the device ordisruptions caused by unexpected changes in the environment.

FIG. 11 is a schematic top planar view of some components of anencoder-equipped crawler robot whose incremental movements can betracked using a dead-reckoning odometry-based process. This device has afour-omni wheel, perpendicular, double-differential configuration. Thistracking device can be connected or mounted to an end effector-equippedcrawler robot. The device shown in FIG. 11 comprises a rectangular frame4 and four double-row omni wheels 4 a-4 d rotatably mounted to frame 4by means of respective axles 6 a-6 d and axle bearings (not shown).Respective encoders 8 a-8 d measure rotation of the omni wheels 4 a-4 d.As the omni wheels roll on a surface, the encoders 8 a-8 d send encoderpulses representing respective encoder counts to the data acquisitiondevice 56 (shown in FIG. 10) after each incremental rotation of eachomni wheel. Each encoder will output an encoder count proportional tothe angle of rotation of a respective omni wheel. These encoder pulseswill be converted to digital data, which the software running oncomputer 48 (shown in FIG. 10) uses to compute the X and Y coordinatesand the heading angle of the tracking device.

In the case of a robotic arm, all of the extendible and rotatablemembers of the robotic arm are typically provided with linear androtational encoders that respectively output pulses representingincremental translation and rotation of those members during operationof the robot, which pulses can be used to provide incremental locationtracking of the robot or a component thereof.

Thus the absolute measurement system (i.e., grid-line detection unit andprojected grid pattern) produces position and orientation data at finitetime intervals. The time interval between successive absolutemeasurements depends on the grid line spacing. This location of therobot component, acquired at periodic intervals when a photo-sensor of agrid-line detection unit crosses successive projected grid lines, can becontinually updated based on subsequent incremental motion measurementsacquired as the photo-sensor moves between those successive grid lines.

In accordance with some embodiments, the method for tracking a locationof a robot component 50 (FIG. 10) of a robotic apparatus comprises:mechanically coupling a grid-line detection unit 16 having multiplephoto-sensors to the robot component 50; mechanically coupling a tool tothe robotic apparatus; projecting light toward the grid-line detectionunit 16, the projected light being in the form of a grid patterncomprising grid lines; moving the grid-line detection unit 16 relativeto the grid pattern; outputting electrical signals from thephoto-sensors having characteristics representing projected light of thegrid pattern sensed by individual photo-sensors during movement of thegrid-line detection unit 16; and tracking a current location of thegrid-line detection unit 16 relative to the grid pattern using digitaldata derived from the electrical signals output by the photo-sensors.

The method described in the preceding paragraph may further comprise:computing a current location of the robot component 50 relative to aframe of reference of a workpiece or part based on at least a locationof the grid pattern relative to the frame of reference of the workpieceor part and a current location of the grid-line detection unit 16relative to the grid pattern; comparing the current location of therobot component 50 to a target location of the robot component 50;causing the robot component 50 to move toward the target location; andusing the tool to perform a manufacturing or maintenance operation onthe workpiece or part while the robot component 50 is located at thetarget location.

In accordance with one embodiment, the tool is a non-destructiveinspection tool that can be used to acquire non-destructive inspectiondata from a portion of the workpiece or part while the robot component50 is located at the target location. In this case, the method furthercomprises: computing a location of the portion of the workpiece or partrelative to the frame of reference of the workpiece or part based atleast in part on a location of the non-destructive inspection toolrelative to the robot component 50; and associating the acquirednon-destructive inspection data with the computed location of theportion of the workpiece or part in a non-transitory tangiblecomputer-readable storage medium.

In contrast to the solid-state device described above, FIG. 12 presentsan isometric view of a rotating grid-line detection unit 16D withmotorized oscillation in accordance with an alternative embodiment. Amotor 38, which is mechanically coupled to a movable robot component,can be operated in a manner that causes the grid-line detection unit 16Dto oscillate/rotate.

FIG. 12A shows a top view of the grid-line detection unit 16D. Thesubstrate 96 of the grid-line detection unit 16D is attached to a distalend of a rotatable output shaft 54 of a motor 38. This embodiment hastwo linear photo-sensors 52 and 54 formed or arranged on the substrate96 in a right-angle pattern for detecting grid lines, while the motor 38rotates the grid-line detection unit 16D back and forth ±45 degrees(passing each time through a 0-degree angular position relative to therobot component) quickly for multiple cycles (e.g., 30 Hz) in the timethat it takes the grid-line detection unit 16D to move from one gridline to the next. As the motor 38 oscillates the grid-line detectionunit 16D back and forth, the linear photo-sensors 52 and 54 detect thepresence of any impinging light from projected grid lines, and at theinstant in which intersecting grid lines 32 and 34 overlie and alignwith the linear photo-sensors 52 and 54 (as shown in FIGS. 12B and 12D),pulses are output to the data acquisition device (see FIG. 10). At thesame time, the relative angle between the motor 38 and the robot isbeing measured with a rotational encoder (not shown in the drawings)mounted on the motor 38. The rotational encoder also outputs pulses tothe data acquisition device. Thus the angular position of the grid-linedetection unit 16D relative to the robot component at the time whenlinear photo-sensors 52 and 54 concurrently output pulses can bedetermined by the computer 48 (see FIG. 10). The pulses output by thelinear photo-sensors 52 and 54 and by the rotational encoder of themotor provide information that enables the computer to track thelocation (i.e., position and orientation) of the grid-line detectionunit 16D in the coordinate frame of reference of the grid pattern.Thereafter the computer can calculate the location of the end effectorof the robot (e.g., a crawler robot, a robotic arm, etc.) in thecoordinate frame of reference of the workpiece or part based on theaforementioned transformation matrix. This configuration allowsacquisition of the X-Y position and heading angle of the robot componentwith just three encoders (i.e., the motor rotational encoder and twolinear photo-sensors) and an oscillating motor.

For example, FIG. 12B shows the grid-line detection unit 16D at aninstant in time when two projected grid lines 32 and 34 are respectivelyaligned with the linear photo-sensors 52 and 54. FIG. 12C shows thegrid-line detection unit 16D after the robot has rotated relative to theprojected grid pattern. It should be understood that the grid lines 32and 34 shown in FIGS. 12A through 12D are stationary during operation ofthe system. FIG. 12C shows the relative angular positions of thegrid-line detection unit 16D and the grid lines 32 and 34 after therobot component (on which the motor of the rotating grid-line detection16D unit is mounted) has been rotated. Any rotation of the robotcomponent cause the 0-degree angular position of the grid-line detectionunit 16D to be rotated relative to the stationary grid lines 32 and 34.Thereafter, the system can determine the new angular position of therobot component in the frame of reference of the workpiece or part bycausing the grid-line detection unit 16D to oscillate/rotate. FIG. 12Dshows the grid-line detection unit 16D at an instant in time duringoscillation/rotation when grid lines 32 and 34 are again respectivelyaligned with the linear photo-sensors 52 and 54. In conjunction with therotational encoder output, the computer can determine the currentlocation of the robot component in the frame of reference of theworkpiece or part.

Disregarding the boundary lines, the grid patterns 30, 40 and 42previously described with reference to FIGS. 3, 4A and 4B respectivelyare uniform in the sense that the pattern of the intersecting verticaland horizontal grid lines does not vary inside the boundary line. Theseuniform grid patterns can be used to provide the system with locationresults in Cartesian coordinates. However, there may be some situationsin which useful results can be achieved for non-Cartesian grid patterns.

FIG. 13 is a diagram representing a top view of a non-uniform projectedgrid pattern 84 that incorporates a guide path 88 for guiding a crawlerrobot or robotic arm (not shown) around a structural feature 86 (on theworkpiece or part) to be avoided. The guide path 88 for guiding therobotic crawler or arm is projected onto the workpiece or part. Thegrid-line detection unit (not shown in FIG. 13) determines theorientation of the robotic crawler or arm relative to the guide path 88.In this way, features to be avoided can be driven around. The gridpattern 84 can be automatically generated using a model or an image ofthe surface to be inspected, with the technician making minor changes tothe track as needed.

In the embodiment depicted in FIG. 13, the guide path 88 comprisesstraight segments and circular segments. The grid pattern 84 furthercomprises a multiplicity of mutually parallel straight vertical gridlines 90 which intersect the straight segments of guide path 88 and amultiplicity of straight radial grid lines 92 which intersect thecircular segments of guide path 88. As the robotic crawler or arm (or aportion thereof) travels along the guide path 88, its distance along theguide path 88 can be determined from the pulses output by photo-sensorsof the grid-line detection unit in response to detection of eachvertical grid line 90 or radial grid line 92 in sequence. The locationresults for this example would not be in Cartesian space, but insteadare a “parametric representation”. In the case depicted in FIG. 13,there is one parameter, which describes the distance along thisnon-branching guide path. If an equation for the curve described inCartesian space is known, then it would be possible to plug in theparametric distance and convert that into a Cartesian location. In someuse cases it might not be necessary to obtain Cartesian coordinates; inothers it would be.

The foregoing is an example of one possible parametric representationthat has one independent variable. To extend this concept further, theprojected grid pattern may comprise a plurality of parallel guide pathsin the direction of travel.

FIG. 14 is a diagram representing a top view of a non-uniform gridpattern 94 that incorporates a plurality of parallel guide paths 88 a-88e for guiding a crawler robot or robotic arm (not shown) around astructural feature 86 (on the workpiece or part) to be avoided. In theembodiment depicted in FIG. 14, each guide path 88 a-88 e comprisesparallel straight segments and concentric circular segments. The gridpattern 94 further comprises a multiplicity of mutually parallelstraight vertical grid lines 90 which intersect the straight segments ofguide paths 88 a-88 e and a multiplicity of straight radial grid lines92 which intersect the circular segments of guide paths 88 a-88 e. Asthe robotic crawler or arm (or a portion thereof) travels along one ofthe guide paths 88 a-88 e, its distance along the guide path can bedetermined from the pulses output by photo-sensors of the grid-linedetection unit in response to detection of each vertical grid line 90 orradial grid line 92 in sequence.

The grid pattern 94 depicted in FIG. 14 would provide two parametricvariables that could be used to provide location long the path—andagain, if the equation for the curves can be described in Cartesianspace, then it would be possible to convert the parametric positioninformation into a Cartesian position.

While systems and methods have been described with reference to variousembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the teachings herein. Inaddition, many modifications may be made to adapt the concepts andreductions to practice disclosed herein to a particular situation.Accordingly, it is intended that the subject matter covered by theclaims not be limited to the disclosed embodiments.

As used in the claims, the term “computer system” should be construedbroadly to encompass a system having at least one computer or processor,and which may have multiple computers or processors that communicatethrough a network or bus. As used in the preceding sentence, the terms“computer” and “processor” both refer to devices comprising a processingunit (e.g., a central processing unit, an integrated circuit or anarithmetic logic unit).

As used herein, the term “location” comprises position in a fixedthree-dimensional coordinate system and orientation relative to thatcoordinate system.

The process claims set forth hereinafter should not be construed torequire that the steps recited therein be performed in alphabeticalorder (any alphabetical ordering in the claims is used solely for thepurpose of referencing previously recited steps) or in the order inwhich they are recited. Nor should they be construed to exclude anyportions of two or more steps being performed concurrently oralternatingly.

The invention claimed is:
 1. A system comprising: a movable targetcomponent; a projector configured to project a stationary grid patterncomprising intersecting grid lines and located to project the stationarygrid pattern toward the movable target component; a grid-line detectionunit mechanically coupled to the movable target component, the grid-linedetection unit comprising at least two photo-sensors, each photo-sensorbeing configured to output a signal when an impinging grid line isdetected; a data acquisition device to convert the signals from thephoto-sensors into signal data in a digital format; a computer systemconfigured to calculate a location of the movable target component basedon the signal data from the data acquisition device; and a motor mountedto the movable target component, the grid-line detection unit beingrotatably coupled to the motor, wherein the at least two photo-sensorscomprise first and second linear photo-sensors which are not parallel toeach other.
 2. The system as recited in claim 1, wherein the computersystem is further configured to control movement of the movable targetcomponent as a function of the calculated location of the movable targetcomponent.
 3. The system as recited in claim 1, wherein the at least twophoto-sensors comprise a first photo-sensor that detects light having afirst color and a second photo-sensor that detects light having a secondcolor different than the first color, and wherein the intersecting gridlines comprise a first multiplicity of grid lines formed by light of thefirst color and a second multiplicity of grid lines formed by light ofthe second color.
 4. The system as recited in claim 1, wherein the gridpattern comprises a multiplicity of boundary grid lines that form aborder surrounding the intersecting grid lines.
 5. The system as recitedin claim 4, wherein the at least two photo-sensors comprise a firstphoto-sensor that detects light having a first color, a secondphoto-sensor that detects light having a second color different than thefirst color, and a third photo-sensor that detects light having a thirdcolor different than the first and second colors, and wherein theintersecting grid lines comprise a first multiplicity of grid linesformed by light of the first color and a second multiplicity of gridlines formed by light of the second color, and the boundary lines areformed by light of the third color.
 6. The system as recited in claim 1,wherein the movable target component comprises a component of a roboticapparatus.
 7. The system as recited in claim 1, wherein the movabletarget component is passive.
 8. A system comprising: a movable targetcomponent; a projector configured to project a stationary grid patterncomprising intersecting grid lines and located to project the stationarygrid pattern toward the movable target component; a grid-line detectionunit mechanically coupled to the movable target component, the grid-linedetection unit comprising at least two photo-sensors, each photo-sensorbeing configured to output a signal when an impinging grid line isdetected; a data acquisition device to convert the signals from thephoto-sensors into signal data in a digital format; a computer systemconfigured to calculate a location of the movable target component basedon the signal data from the data acquisition device, wherein theintersecting grid lines comprise a guide path grid line and amultiplicity of grid lines intersecting the guide path, wherein theguide path grid line comprises straight segments and curved segments. 9.The system as recited in claim 8, wherein the computer system is furtherconfigured to control movement of the movable target component as afunction of the calculated location of the movable target component. 10.The system as recited in claim 8, wherein the at least two photo-sensorscomprise a first photo-sensor that detects light having a first colorand a second photo-sensor that detects light having a second colordifferent than the first color, and wherein the intersecting grid linescomprise a first multiplicity of grid lines formed by light of the firstcolor and a second multiplicity of grid lines formed by light of thesecond color.
 11. The system as recited in claim 8, wherein the gridpattern comprises a multiplicity of boundary grid lines that form aborder surrounding the intersecting grid lines.
 12. The system asrecited in claim 11, wherein the at least two photo-sensors comprise afirst photo-sensor that detects light having a first color, a secondphoto-sensor that detects light having a second color different than thefirst color, and a third photo-sensor that detects light having a thirdcolor different than the first and second colors, and wherein theintersecting grid lines comprise a first multiplicity of grid linesformed by light of the first color and a second multiplicity of gridlines formed by light of the second color, and the boundary lines areformed by light of the third color.
 13. The system as recited in claim8, wherein the movable target component comprises a component of arobotic apparatus.
 14. The system as recited in claim 8, wherein themovable target component is passive.
 15. A method for tracking alocation of a movable target component, the method comprising:mechanically coupling a grid-line detection unit having multiplephoto-sensors to the movable target component; projecting light towardthe grid-line detection unit, the projected light being in the form of astationary grid pattern comprising intersecting grid lines; moving thegrid-line detection unit relative to the stationary grid pattern;outputting electrical signals from the photo-sensors havingcharacteristics indicative of projected light of the grid pattern sensedby individual photo-sensors during movement of the grid-line detectionunit; tracking a current location of the grid-line detection unitrelative to the stationary grid pattern using digital data derived fromthe electrical signals output by the photo-sensors; and computing acurrent location of the movable target component relative to a frame ofreference of a workpiece or part based on at least a location of thegrid pattern relative to the frame of reference of the workpiece or partand a current location of the grid-line detection unit relative to thegrid pattern.
 16. The method as recited in claim 15, further comprising:comparing the current location of the movable target component to atarget location of the movable target component; and causing the movabletarget component to move toward the target location.
 17. The method asrecited in claim 16, further comprising: mechanically coupling a tool tothe robotic apparatus; and using the tool to perform a manufacturing ormaintenance operation on the workpiece or part while the component ofthe robotic apparatus is located at the target location.
 18. The methodas recited in claim 15, further comprising: equipping the movable targetcomponent with a non-destructive inspection probe; using thenon-destructive inspection probe to acquire non-destructive inspectiondata from the workpiece or part while the movable target component is atits current location; and mapping the acquired non-destructiveinspection data to location data representing the current location ofthe movable target component in a non-transitory tangiblecomputer-readable medium.
 19. The method as recited in claim 15, whereinthe intersecting grid lines comprise a first multiplicity of grid linesformed by light of a first color and a second multiplicity of grid linesformed by light of a second color different than the first color. 20.The method as recited in claim 15, wherein the intersecting grid linescomprise a guide path grid line and a multiplicity of grid linesintersecting the guide path, wherein the guide path grid line comprisesstraight segments and curved segments.